Multivibrator



July 20, 19.54 Q w GRAY AETAL 2,684,442

MULTIVIBRATOR Filed July 51, 1951 ATi'oRNEY Patented July 20, 1954MULTIVIBRATOR George W. Gray, Lambertville, and Arthur S.

Jensen, Princeton, N. J., assignors to Radio Corporation ci America, acorporation of Delaware Application July 31, 1951, Serial No. 239,508

The terminal fteen years of the term of the patent to be granted hasbeen disclaimed Cl. Z50- 27) 4 Claims.

This invention relates to multivibrators and more particularly to animprovement in multivibrators of the unistable or slideback type.

Slideback or unistable multivibrators are often used to generate uniformwidth and amplitude pulses in response to random pulses of nonuniformwidth ano'. amplitude. These multivibrators or trigger circuits are ofthe well known type employing two electron discharge tubes in which thegrids and anodes are cross-connected to each other. Bias is applied tothe tubes so that in the standby or stable condition, one of them isconducting and the other is non-conducting. A pulse is usually appliedto the conducting tube to render it non-conducting. This transfersconduction to the other tube. This unstable condition is maintained fora time determined by the time constant of the coupling between thetubes. Then the tubes return to their stable condition of conduction. Anoutput is usually derived from the anode of the other tube, or the onewhich is non-conducting in the standby state.

In attempting to use a slideback trigger circuit of the general typedescribed to drive directly a substantially low impedance load,diiliculty was experienced. The trigger circuit behaved in typicaloverloaded fashion by providing a low output with an extremely poorwaveshape. Reducing the plate load of the output resistor merely reducedthe output, which was not a very eilicient practice. Or" course, abuffer stage could have been used as an impedance converter, but `thisis not as economical a practice as is desired.

It is, therefore, an object of this invention to provide a novel andimproved slideback trigger circuit. to provide a novel slideback triggercircuit which can drive a substantially low impedance load.

it .is a further object of this invention to provide an improvedslideback trigger circuit which permits circuit economies by providing asuiciently high output, thus eliminating the need for an amplificationstage.

These and other objects of the present invention are achieved byproviding a slideback trigger circuit having two tubes wherein the anodeoi' a first tube is directly connected to the grid of a second tube, abias is applied to the grid of Athe nrst tube to maintain it conductingin the standby condition, and a bias is applied to the second tubecathode to maintain it at cutoi in .the standby condition. The value ofthe anode load resistor of the rst tube is chosen to permit the secondtube grid to rise in potential substantially above its cathode potentialeven though It is a further object of this invention this means drawinggrid current when the rst tube is rendered non-conducting and the secondtube conducting. The second tube anode load resistor is chosen to have avalue which'is low because when driven to the unstable condition thesecond tube plate voltage will drop nearly to cathode potential whetherthe load resistor is large or small; thus the smaller resistor gives thesame amplitude output across a lower impedance.

The novel features of the invention as well as the invention itself,both as to its organization and method of operation, will best beunderstood from the following description, when read in connection withthe accompanying drawing of a circuit diagram of an embodiment of theinvention.

Referring now to the drawing, there may be sec-n a circuit diagram of anembodiment oi the invention. It comprises a rst electron discharge tubeIt having an anode I2, grid It, and cathode I6, and a second electrondischarge tube 2E! also having an anode 22, grid 24, and cathode 25. Theanode I2 of the first tube IG is directly coupled to the grid 24 of thesecond tube 20. The grid I4 of the iirst tube I0 is connected to theanode 22 of the second tube 20 by means of a coupling condenser 30. Asource of operating potential of B+ is provided. An anode load resistorI8 is connected between the anode I2 of the iirst tube and the source ofoperating potential. Therst tube cathode I6 is connected to ground.

Negative input pulses are applied to the grid of the rst tube I 0through a diode 32. The rst tube grid I 4 is also connected to itscathode I6 through a diode 34 whose anode 36 is connected to the firsttube grid I4 and cathode 33 is connected to the first tube cathode I6.This diode 34 is maintained conducting by means of a resistor 40connected from the iirst tube grid I4 to the source of operatingpotential. Since a negative pulse is applied to the iirst tube grid toeffectuate a transfer of the circuit from a stable to an unstable state,in order to permit the first tube grid to go positive again and thuspermit return to the stable condition, electrons must be removed fromthe grid. This resistor 40 connected to the source of B+ serves thatpurpose and restores the diode 34 to conduction after the conduction hasceased by reason of the application of a negative pulse.

The anode 22 of the second tube 20 is connected to the source of B-lbyan anode load resistor 28. A voltage divider 42 is connected across thesource of operating potential and includes a variably tapped portion 44which is connected to the cathode 26 of the second tube. A capacitor isconnected from the cathode of the second tube to ground. rihe capacitord6 serves to maintain constant the bias applied to the cathode of thesecond tube. rThe voltage divider is tapped to provide cutoff bias forthe second tube. This occurs when the cathode bias exceeds the voltageapplied by the first tube anode to the second tube grid.

The diode 34 connected between the first tube grid and cathode inconjunction with the resistor connected to the B+ source serves tomaintain the first tube in a conducting condition. It should be notedthat the diode may be omitted with some tube types and satisfactorydiode action is still obtainable from the grid-cathode path of the tube.The use of the diode insures this action. The application of a negativepulse to the imput diode 32 causes the first tube l0 to be cut off. Thisapplies a positive pulse from the nrst tube anode l2 to the second tubegrid 2li causing the second tube to become conductive. The amplitude ofthe trigger pulse need only be big enough to initiate the action, sinceas soon as tube 2i) starts to conduct, feedback will complete theswitch. The nrst anode load resistor i8 has its value selected lowenough so that even though the grid of the second tube draws some gridcurrent, the grid is driven considerably positive, substantially abovethe tube cathode potential. The second anode load resistor 28 is alsoselected rather low. As a result of the large positive signal on thesecond tube grid, the second tube draws a large plate current andaccordingly its output is of a large amplitude across a rather lowimpedance. on the RC time constant of the coupling from the second tubeplate 22 to the first tube grid Ui, the grid of the first tube, whichhas been negative in order to hold the tube non-conducting, becomespositive enough to cause the circuit to switch back to its normaloperation.

By eliminating the coupling condenser between the first and second tubesnot only is the tube grid permitted to be driven so positive that thetube is forced to draw an extremely large i plate current, but also nocoupling condenser is charged up by the grid current so as to introducea dead time following each output pulse. This type of operation is notpossible with the usual coupling condenser present, since a large gridcurrent could not be drawn without causing blocking by the couplingcondenser. The output of the second tube is increased two to threetimes. As is usual, such output is derived from the anode oi the secondtube. As an illustration of the circuit values used in a typicalembodiment of the invention which was built and operated, but not toserve as any restriction thereon, the following values are provided:

After a time, depending -(50 milliamperes in the illustrated case), theduty cycle of the tube should be chosen as a rather low one in order toprevent the tube from suffering damage. In view of the low value of thesec- CFL i ond anode load resistor, a substantially low impedance loadcould be readily driven.

In the above description, there has been described and shown a simple,novel, slideback type of trigger circuit which is capable of generatinga large output and can supply this output to a low impedance load.

What is claimed is.

i. A slideback trigger circuit comprising first and second electrondischarge tubes each having an anode, a cathode and a grid electrode,means coupling said first tube grid to said second tube anode, saidsecond tube grid being directly connected to said first tube anode, afirst anode load resistor connected to said first tube anode, a secondanode load resistor connected to said second tube anode, means to biassaid rst tube grid to maintain said first tube conducting when saidslideback trigger circuit is in standby condition, means to apply a biasto said second tube cathode to maintain said second tube non-conductingwhen said slideback trigger circuit is in a standby condition, means toapply a pulse to said nrst tube grid to transfer conduction from saidfirst to said second tube, the last named condition of conduction beingreversed after a time determined by the time constant of said meanscoupling said first tube grid with said second tube anode, and means toderive an output from said second tube anode, said means to bias saidrst tube grid including a rectifier having an anode and cathode, saidrectiiier anode being connected to said first tube grid, said rectifiercathode being connected to said rst tube cathode, a resistor connectedto said first tube grid, and means to apply a positive voltage to saidrectiiier anode through said resistor.

2. A slideback trigger circuit comprising first and second electrondischarge tubes each having an anode, a cathode and a grid electrode,said second tube grid being directly connected to said nrst tube anode,means coupling said first tube grid to said second tube anode, a sourceof operating potential, a first anode load resistor connected betweensaid source and said first tube anode, a second anode load resistorconnected between said source and said second tube anode, a rectifierhaving an anode and a cathode, said rectifier anode being connected tosaid first tube grid and said rectifier cathode being connected to saidfirst tube cathode, means in circuit with said source and said rectierto render said rectifier conducting and to supply to said rst tube grida voltage to render said first tube conducting when said slidebacktrigger circuit is in a standby condition, said first anode loadresistor having its resistance value selected sufficiently low to permitthe potential of said second tube grid to rise substantially above thepotential of said second tube cathode when said iirst tube is renderednon-conducting, said second anode load resistor is selected to have alow value of resistance when compared with said rst anode load resistor,means to apply a voltage to said second tube cathode that is positivewith respect to said second tube grid to maintain said second tubenon-conducting when said slideback trigger circuit is in a standbycondition, means to apply a negative pulse to said first tube grid totransfer conduction from said first to said second tube grid for a timedependent upon the time constant of said means coupling said iirst tubegrid to said second tube anode, and means to derive an output from saidsecond tube anode.

3. A slideback trigger circuit comprising first and second electrondischarge tubes each having an anode, a cathode and a grid electrode, acoupling capacitor connecting said first tube grid to said second tubeanode, said second tube grid being connected directly to said rst tubeanode, a source of operating potential, a first anode load resistorconnected between said source and said first tube anode, a second anodeload resistor connected between said source and said second tube anode,a Voltage dropping resistor connected between said source and said rsttube grid, a rectier having its anode connected to said rst tube gridand its cathode connected to said rst tube cathode, said rst tube beingin a state of conduction when said slideback trigger circuit is in astandby condition, a Voltage divider connected across said source ofpotential, means coupling said second tube cathode to said voltagedivider at a point to derive a bias voltage to maintain said second tubenon-conducting when said slideback trigger circuit is in a standbycondition, a capacitor connected between said second tube cathode andthe low potential end of said voltage divider, means to apply a negativepulse to said rst tube grid to transfer conduction from said first tosaid second tube for a time proportional to the value of said couplingcapacitor, and

means to derive an output from said second tube anode.

4. A slideback trigger circuit as recited in claim 2 wherein theresistance value of said second anode load resistor is selected to havea 10W value when compared with said first anode load resistor, and saidrst anode load resistor has its resistance value selected sufficientlylow to permit the potential of said second tube grid to rise above thevalue of the potential of said second tube cathode when said rst tube isrendered non-conducting.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 2,183,966 Lewis Dec. 19, 1939 2,363,659 Doba Nov. 28, 19442,410,920 Atwood, J1' Nov. 12, 1946 2,412,485 Whiteley Dec. 10, 19462,524,691 Bliss Oct. 3, 1950 2,595,646 Doba et al. May 6, 1952 FOREIGNPATENTS Number Country Date 863,344 France Jan. 2, 1941

